In fuel burner control systems that operate by monitoring the infrared radiation of a flickering flame, problems have arisen where the system is fooled by the infrared radiation of the hot refractory material in the furnace and a shimmering effect caused by the movement of air currents or unburnt fuel. The movement of unburnt fuel and/or air causes a shimmer or flickering to appear at the flame detector which simulates the presence of a flame and the system can thus fail in an unsafe manner.